Low-Dynamic-Range images (LDR images) are images whose luminance values are represented with a limited number of bits (most often 8 or 10). Such a representation does not render correctly small signal variations, in particular in dark and bright luminance ranges. In high-dynamic range images (HDR images), the signal representation is extended in order to maintain a high accuracy of the signal over its entire range. In HDR images, pixel values are usually represented in floating-point format (either 32-bit or 16-bit for each component, namely float or half-float).
The most popular high-dynamic-range file format is openEXR half-float format (16-bit per RGB component, i.e. 48 bits per pixel) or openEXR integer format with a long representation, typically at least 16 bits.
It is known for encoding a HDR image to reduce the dynamic range of the image in order to encode the image with a legacy encoder (initially configured to encode LDR images). In the document from Larson entitled “LogLuv encoding for full gamut, high dynamic range images” published in 1998 in Journal of Graphics Tools, the HDR images are represented in a LogLuv color space. The luminance and chrominance components are then processed by a JPEG2000 encoder in the same way as classical LDR images. Wavelet based encoders such as JPEG2000 encoders are widely used in the digital cinema industry. The LogLuv is a color space used for representing HDR images but is not well suited for compression.